1. Field of the Invention
This invention relates to a process and apparatus for representing digital data by binary signals, in which the data are in the form of binary-coded, ternary-coded characters, or characters coded in a higher value code, and in which the characters are fed, in the form of data signals, to a data transmitter which assigns binary valves to the characters, and in which the binary signals are transmitted over a transmission channel to a data receiver which regains the digital data from the binary signals.
2. Description of the Prior Art
The recording processes preferably utilized for the storage of digital data are those processes in which the digital data are assigned rectangular binary signals which assume only two amplitude values. Each of these two amplitude values are assigned a code state, normally a saturation state of a storage medium, for example the magnetization states of a magnetic layer.
For the storage of data on the storage medium, a plurality of processes have already been provided for recording data in the form of binary signals. Known recording methods are, for example, the non-return to zero (mark) or NRZ (M) method and the phase encoding or phase modulation method. In the non-return to zero (M) method, only one of the two binary characters is represented by one single change in the code states. In the phase modulation method, each zone of a track assigned to a binary character, and known as a track element, is divided into two parts, each of which corresponds to one of the two code states. Each change in direction of the magnetization state is permanently assigned to one of the two binary characters. In this method, in the event of successive identical binary characters, additional changes occur at the boundaries of the track elements.
Another known recording method is disclosed in the U.S. Pat. No. 3,414,894, which is also known as "MFM" recording. In this method, the binary character 1 is represented by a change in the code state in the middle of a track element, and an additional change occurs at the boundary between two track elements having the binary character 0.
These known recording methods can also be utilized for the transmission of data. In this case, the amplitude values are assigned as code states, for example to the pluralities of the direct current. The known recording methods have the disadvantage, however, that the representation of each binary character requires at least one change in the code state of the storage medium, or of the transmitted quantity. If the storage density is defined as the number of binary characters which may be stored per unit of length, then the length unit is the smallest occurring interval between two changes, the maximum storage density which is obtained in the known methods is one bit per unit length. Consequently, the data transmission is subject to a maximum transmission speed of one bit per element duration, if the element duration is the smallest interval of time between two changes in the code states.